To enable a turbojet engine to operate efficiently, it is necessary that the air entering its compressor have substantially uniform velocity, pressure, and direction of flow around the entire circumference of the inlet. It is of special importance that all of the blades of the first stage of the compressor encounter air that is moving at nearly the same velocity. In normally adequate inlets, the desired uniform pressure, velocity, and flow direction have not been obtained across the inlet at times when the inlet directions were oblique to their longitudinal axes, that is, during subsonic flight at high angles of attack or in a yaw for example.
In some of the prior art intake ducts, there are wall portions that define opposite interior surfaces which extend parallel to the longitudinal axis of the aircraft. One of these surfaces becomes a lee surface and the other a weather surface when the aircraft moves obliquely to the longitudinal axis of the duct, and the airstream moving into the duct has to curve through an angle corresponding to the angle between the longitudinal axis of the aircraft and its direction of flight. The result is that there are differences in velocity and in pressure in the air at different points across the cross section of the curving stream flow.
Boundary layer air accumulates on the wall surface ahead of an inlet. Because of this accumulation of boundary layer air, the downstream air at a compressor inlet has one zone in which its velocity and stagnation pressure are substantially lower than the average velocity and pressure of the air moving to the compressor inlet. In another zone, circumferentially spaced from the first, the velocity and stagnation pressure of the air are higher than the average. The result is that the compressor blades, as they rotate, pass through the zones successively, alternately encountering air moving at high relative velocity and at low relative velocity. This circumferentially nonuniform distribution encourages the stalling of the blades and causes a cyclic pressure variation in the compressor, bringing about poor functioning and compacity of the engine and possible flameout.
In the prior art both parallel wall and diverging wall flush inlets have been studied. These inlets had right angle corners where the sides meet the skin of the aircraft and where they meet the ramp of the inlet. These sharp corners were used intentionally because they result in low energy boundary layer air being directed out of the inlet, it being important that a minimum amount of low energy air be allowed to enter the inlet. However, it was found that in these inlets problems were experienced when they were yawed with respect to the free stream direction. When yawed the inlets produced large amounts of nonuniform flow at the engine compressor face, resulting in poor engine performance.
In a low flying aircraft such as a cruise missile, which flies 200 feet off the ground, for example, there is an additional problem. That is, it is desired that the missile be not easily detected by radar and the typical protruding inlet presents the problem that it provides good radar return which is unsatisfactory in a cruise missile. The so-called flush or submerged inlet lowers radar return in that it has a low radar cross section.